1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display device. More particularly, the present invention relates to a crystallization pattern for forming a multi-crystalline silicon thin film transistor and a method for crystallizing amorphous silicon using the same.
2. Description of the Prior Art
As generally known in the art, a thin film transistor (TFT), which is used as a switching device in a liquid crystal display device, is the most important part for performance of the liquid crystal display device. Herein, mobility or current leakage, which is the basis for determining performance of the TFT, significantly relates to a state or a structure of an active layer, which is a movement path of a charge carrier. That is, the mobility or current leakage may vary depending on the state or the structure of a silicon thin film forming the active layer.
In the case of a liquid crystal display device, which is currently used in the field, an active layer of a TFT mainly includes amorphous silicon (a-Si). However, an a-Si TFT including the active layer made of a-Si represents low mobility of about 0.5 cm2/Vs, so there are limitations in fabricating all switching devices used in the liquid crystal display device by using the a-Si TFT. That is, a driving device used for driving a peripheral circuit of the liquid crystal display device must operate at a high speed, but the a-Si TFT cannot satisfy the operational speed required for the driving device for the peripheral circuit. Thus, the a-Si TFT is not suitable for realizing the driving device for the peripheral circuit.
In contrast, a polycrystalline silicon (poly-Si) TFT including an active layer made of poly-Si represents high mobility of about several tens to hundreds of cm2/Vs, so the poly-Si TFT can be operated at a high speed suitable for the driving device for the peripheral circuit. For this reason, if a poly-Si layer is formed on a glass substrate, it is possible to easily provide a pixel switching device in addition to driving parts for the peripheral circuit.
The poly-Si thin film cannot be directly formed on the glass substrate. Thus, it is necessary to crystallize a-Si in order to form the poly-Si thin film on the glass substrate. In order to crystallize a-Si, there have been suggested an excimer laser annealing (ELA) scheme using a laser, and sequential lateral solidification (SLS) scheme for sequentially irradiating a patterned laser onto a substrate being moved. Such ELA and SLS schemes have been successfully commercialized.
According to the ELA scheme, a laser having low energy causing partial melting is repeatedly irradiated onto a substrate while the substrate is being moved in such a manner that 90% of the laser can be overlapped on the substrate, thereby obtaining a poly-Si thin film. However, since the ELA scheme is performed while repeatedly irradiating the laser onto the same location about 10 to 20 times, productivity is significantly lowered. In addition, since the maintenance period for laser equipment is short, the maintenance cost thereof may increase.
According to the SLS scheme, a laser is irradiated onto an a-Si thin film through a mask formed with a slit pattern, thereby completely melting the a-Si thin film in such a manner that the lateral growth may occur from both boundary lines of the solid phase and the liquid phase. Such a lateral growth continuously occurs when the laser is repeatedly irradiated, so that a poly-Si thin film having large-size grains can be obtained. In the case of the SLS scheme, in particular, in the case of a 2-shot SLS scheme, which has been commercialized, it is sufficient if laser irradiation is performed with respect to the a-Si thin film by two times, so the SLS scheme has a process window larger than that of the ELA scheme. In addition, the SLS scheme can obtain small-size grains having high quality corresponding to single crystal, so the SLS scheme is variously used.
However, when crystallizing a-SI through the SLS scheme, the characteristic of the TFT may be changed depending on the position and number of grain boundaries included in a TFT channel. In particular, in view of an array substrate, TFT characteristics may be irregular depending on pixels.
Therefore, it is very important to obtain uniform TFT characteristics when employing the SLS scheme. In order to solve this problem, there have been suggested various conventional schemes, such as a tilted SLS scheme, a pixel mixing scheme, a scheme using a bending type gate, a scheme for irradiating the laser on a-Si after forming a metal pattern having a reflection characteristic on the a-Si thin film (Korean Patent Application No. 1996-080083), and a scheme for irradiating the laser on a-Si after forming an anti-reflection layer pattern, such as an insulating layer, on the a-Si thin film (Korean Patent Application No. 1996-050488).
Although it is not illustrated or described in detail, as generally known in the art, the tilted SLS scheme can solve ununiformity of the TFT characteristics derived from the grain boundaries by varying the tilt angle. However, if the tilt angle is too large, a non-crystallized region may exist, thereby lowering the efficiency of the crystallization process. In addition, in the case of the pixel mixing scheme, productivity is reduced and the process is complicated as the mixing degree is increased.
In the case of the scheme using the bending type gate, which is under discussion, a wasted space may exist due to the structure of the bending type gate, so that the aperture of a pixel section may be reduced, space utility at a peripheral circuit may be lowered, and a design of the bending type gate may be complicated.
In addition, in the case of the scheme using the metal pattern, the metal may cause contamination. In the case of the scheme using the pattern, such as the metal pattern and the anti-reflection layer pattern, it is difficult to deal with devices having various sizes. In particular, in the case of the crystallization scheme using the pattern, the crystallization process proceeds from an outer lateral side of an a-Si pattern toward an internal region of the a-Si pattern, so that poly-Si having large-size grains is grown in a region located within a predetermined distance from the outer lateral side of the pattern, and poly-Si having small-size grains is grown in a region located away from the outer lateral side of the pattern by more than the predetermined distance, thereby causing irregular poly-Si. Thus, this scheme also represents limitations in use.